POSITRON-ANNIHILATION STUDY OF DISLOCATIONS PRODUCED BY POLISHING IN THE SURFACE OF IRON SINGLE-CRYSTALS - .1. DENSITY PROFILE AND REMOVAL BY ANNEALING

The trap concentration as a function of the depth from the abraded surface of iron single crystals was investigated by means of positron Doppler broadening. The deformation depth was about 40 μm from the surface when the specimen was mechanically polished on 240-grit SiC paper. The depth of the damaged region is less than 15 μm from the surface when a slurry of 0.3 μm Al2O3 particles is used. A mixture of edge and screw dislocations was found in the deformed layer. The two stages in recovery observed upon annealing mechanically polished iron are interpreted as being due to the disappearance of screw components during lower temperature anneals (273 to 473 K). When the number of screw dislocations has fallen considerably, the edge-dislocation dipoles begin to disappear. The activation energies for recovery of screw and edge dislocations are estimated at 30.5 and 103.2 kJ/mol, respectively, using the second-order Li model. Based on these small energies, dislocation dipoles disappear by glide rather than by climb in pure iron. Migration of monovacancies and climb are apparently important with impure iron, since the activation energy is similar to that for vacancy migration.